1. Field of the Invention
The invention relates generally to the field of determining and displaying geologic stress information. More particularly, the invention relates to display and interpretation techniques for geologic stress information.
2. Background Art
Forces acting in specific directions (“stresses”) on subsurface rock formations are important to determine in planning subsurface geologic operations such as mining and wellbore drilling, for example. Knowledge of the stresses may enable planning the geologic operations to avoid undesirable events, for example, wellbore and/or wellbore casing collapse, formation blowout, loss of drilling or cementing fluid and production into a wellbore of formation solids.
In determining the stresses acting on any particular formation at a particular depth in the subsurface, in most cases it can reasonably assumed that the vertical stress is a principal stress, meaning that no shear stress exists in the plane perpendicular to the direction of Earth's gravity, and that the maximum and minimum horizontal stresses are orthogonal to each other. The foregoing assumptions enable description of the total in situ stress tensor with four values: Sv (vertical stress); SH (maximum horizontal stress); Sh (minimum horizontal stress); and SH azimuth (geodetic direction of the maximum horizontal stress); For other cases where Sv is not a principal stress, the total in situ stress tensor can be described by six values: Sv′ (the principal stress that is closest to the vertical direction); SH′ (the larger of the other two principal stresses); Sh′ (the remaining principal stress); Sv′ deviation (represented by DEV and which is the geodetic direction of Sv′ from vertical); Sv′ azimuth (represented by AZI and which is the geodetic direction of Sv′); and SH′ azimuth [AZIH] which is the geodetic direction of SH′).
An important characteristic of geologic stress is believed to be the relative magnitudes of the three principal stress components (Sv, SH and Sh, or Sv′, SH′ and Sh′). Relative magnitudes of the three principal stresses are related to the stress regime and to the type of rock failure induced by the stresses, for example, faults at the geologic structure scale and fracture type at the rock formation layer scale. In determining the likely failure mode of rock formations, it is believed that the relative magnitude of each principal stress component is more important than the absolute values, which vary mostly as a function of depth. The orientation of the stress (SH azimuth, or Sv′ deviation, azimuth and SH′ azimuth) and the fluid pressure in the pore spaces of porous subsurface formations (“pore pressure”) (Pp) are also important to predict the type of failure to expect in subsurface rock formations.
Because it is believed that the relative magnitudes of the principal stresses is more important than their absolute values, it is desirable to have a display and interpretation technique that takes advantage of the foregoing belief to enable better interpretation of stress information.